Here is a simple IR audio link that can be used to transmit audio signals up to 4 meters. The signal to be transmitted is applied to the base of Q1 via resistor R4.The transistor Q1 drives the IR transmitting diodes D1 and D2.The audio input will be modulated to the IR signals transmitted.
The transmitted IR signals will be picked by the photo transistor Q2.The emitter voltage of the transistor Q2 will change according to the sound modulated to the IR signal. The transistors Q3 and Q4 amplifies this signal to drive the speaker or headphone.C1 and R3 forms a filter to avoid interference from stray IR signals.

Circuit diagram with Parts list.

Notes

The circuit can be assembled on a general purpose PCB.

Use 9V PP3 batteries for powering the transmitter and receiver.

The phototransistor Q2 can be any NPN phototransistor like PNZ154, PNA1605F, BPW77NA or BPW85.

Hi,I want to use this transmitter and receiver to send a data,data is not sound and is straem of ones and zeros bits. is it possible?! and is it possible to use 5 volt for power.beacause I want to use it in usb system.thanks for ur answer.

1) 1st broken a popo3 9volt battery for getting the lied of jack point of popo3 battery
2) solar wire with this jack which u got by broken the battery and out a positive lid and a negative then
3) other end of the were solder the wire into circuit ,
4) and connect with another popo3 battery which this new jack which u made by broken and soldering wire by a popo3 battery

i have mini project and i used simple RF with IR led’s ,, ihave a question regard the 2_ir
why they are connected in this way ? i mean there is one photo transistor
the second question how i can simulate this circuit using pspice program
thank you

If i do not have Transistor BC 548. What kind of transistor can i replace it with?
One more question, what kind of filter are you using in this project?
How can we compute the cutoff frequency of this filter?

This circuit is garbage. Q1 is not biased at all. If the Audio IN signal is AC, Q1 will only turn on for those portions of the signal that exceed 0.7V, resulting in severe distortion.

Whoever designed this must think that transistors are self-biasing amplifiers, like op-amps: just add power through a load, and your input, and it’s good to go.

Another problem is that Q1 has no emitter resistor at all. The circuit is aiming for maximum gain. Unfortunately, not only does that mean lots of nonlinearity, but also suboptimal input impedance. Basically the input impedance is just the 1K resistor plus the base-emitter junction’s ESR. That may be too low to be driven by some sources.

Hi Kiran it is from the transmitter. the current through the standard market IR LED is around 10mA, through R1 there will be a drop of 10volts which is not possible hence low current will flow through LEDs and Q1. Each LED requires 1.3 volt at 10mA a drop of 4 volt is required. 4 / 0.01 is 400 ohms reduce R1 to 390 ohms and add a resistance of 22K between collector and base of Q1. add an isolation capacitor of 0.22uF ceramic disc from input to Q1 base. It looks your audio input is of very low impedance you are facing the distortion problem. The above modifications will cater to your requirement.

Hi Kiran see the notes it indicates clearly that you can use any NPN photo transistor. There is nothing called most powerful photo transistor, for higher sensitivity use darlington photo transistor with additional external optical lens.

Hi Kiran to avoid line of sight problem,use RF transmitter and receivers to control your helicopter (normally the rc is supplied as part of equipment by the toy supplier and not available in the free market).

dear sir,
i wanted to apply this idea to make a transmitter and receiver for my micro rc helicopter which i have made.Can you please help, I think we have to slightly change the circuit.Can you please tell where and what to connect.

Hi Virender if you have a old optical mouse from which you use. or you can prepare one by using a bc108 or 2N2221. by filing the edge of top portion of the transistor remove the top disc. clean internals with pure alcohol dry and then fix a small transparent plastic disc in the top hole with suitable adhesive. you photo transistor is ready.

Hi all, can anyone please let me know how to change Q2 in the receiver side. I could not find any NPN phototransistor like PNZ154, PNA1605F, BPW77NA or BPW85 in our area. Any reply shall be highly appreciated.
Plase mail me to :vrndrsngh85@gmail.com. Thanx in advance.

Dear sir, can u plz. let me know how to replace Q2 phototransistor with IR LED. I mean I want to use IR LED by replacing phototransistor in IR Audio Link Circuit. It is possible? Any reply shall be highly appreciated. With thx.

Hi Rajesh D1 & D2 are IR diodes. You have to take care of anode and cathode. transmitter and receiver should see each other line of sight is important.
This circuit transmits mono signal only. For stereo encoder and decoders are required. The principle of operation is the communication through modulated IR signal. The audio source modulates the IR LEDs they transmit modulated IR beam. The IR transistor in the receiver receive the signal it is demodulated and amplified by Q3 and Q4. For stereo a simple way will be use two identical circuits for each channel. use polarizing glasses for each channel transmitter and receiver. Use vertical polarization for one channel and horizontal for the other.

Hai,
is this circuit transmitt mono signal or stereo signal? If it is mono how can i make this stereo. What is the input voltage of audio signal in transmitter? Can you tell me how is this circuit transmitt and receives audio signals? What is the principle of working of this circuit?

Hello seetharaman, I want to use this circuits in some different purposes – like Remote ON/OFF LED bright Light circuit . I want to use TV Remote in this purposes . If I pressed one time in any button of TV Remote then LED will glow and again pressed remote button LED will be OFF . I want to make this circuit with photo transistor . Please don’t say to use CD4017 and Relay . Because I cann’t use it . Because I have a LED light which have 20 LED’s and 4V 0.5Ah Battery which casing too small to establishe Relay and IC there .

Please tell me how can I make such circuit with photo transistor ?

And about this circuit – It for mono only ! How can I make it for stereo ? I don’t understand why most designer design their audio circuit as mono !! What is the problem with designer to make stereo ?? I really don’t understand !!

Hi Mohin the output of this circuit, to be connected to a bistable such that its output will go high with one push and with second push it will go lo.
To make the above circuit suitable for stereo it will become a dual channel transmission, then you require an encoder to multiplex both the channel signals for sending and a decoder at the receiving end to convert into discrete channels. this will make the circuit complicated as there is no suitable IC available in the market for the same. we will try to develop a discrete circuit in the future.

Hello seetharaman, I want to use this circuits in some different purposes – like Remote ON/OFF LED bright Light circuit . I want to use TV Remote in this purposes . If I pressed one time in any button of TV Remote then LED will glow and again pressed remote button LED will be OFF . I want to make this circuit with photo transistor . Please don’t say to use CD4017 and Relay . Because I cann’t use it . Because I have a LED light which have 20 LED’s and 4V 0.5Ah Battery which casing too small to establishe Relay and IC there .

Please tell me how can I make such circuit with photo transistor ?

And about this circuit – It for mono only ! How can I make it for stereo ? I don’t understand why most designer design their audio circuit as mono !! What is the problem with designer to make stereo ?? I really don’t understand !!

Hi Musthafa Audio input is fed to IR transmitter and the sound is transmitted as modulated IR ray. This ray is sensed by the photo transistor Q2 and converted back to audio signal. the out put can be suitably amplified and listened. This is a cable less audio link.

I am very much interested in tracing circuits, though I do not build them. I like to see each and every circuit that I come across. I thank Mr.John for his circuits. Can I give some ideas, as expect any one in your group build them for me? kindly reply.